Onlookers in Beijing, China, keep a safe distance from a giant sinkhole that opened in the middle of a busy street last Tuesday, swallowing a truck. Several news reports say the sinkhole formed above a tunnel being dug for construction of a subway line. (See pictures of a huge sinkhole in Guatemala City.)
But Chinese engineers urged caution in jumping to conclusions: "It could have resulted from multiple causes," Wei Jinglian, a senior engineer from the Beijing Institute of Geology, told China's Global Times. "Water leakage from old pipes underground may have softened the soil," Jinglian said, "or there has been too much pressure frequently on the same section of road."
Mining and construction activities can weaken the subsurface and cause sinkholes to collapse, said Mike Hoyal of the Tennessee Division of Geology. But sinkholes can also form naturally when water-saturated soil becomes too heavy, causing the roofs of existing voids and caves in the ground to collapse.
In addition, acidic rain or groundwater can enlarge a natural fracture in a limestone bedrock layer to form a sinkhole. The water "dissolves out the calcium carbonate in the limestone and forms fractures," Hoyal said. As the crack gets bigger, the topsoil can gently slump, exposing the sinkhole to the world.